From K6[Re6−xMoxS8(CN)5] Solid Solution to Individual Cluster Complexes: Separation and Investigation of [Re4Mo2S8(CN)6]n− and [Re3Mo3S8(CN)6]n− Heterometallic Clusters

A series of new cluster compounds with {Re4Mo2S8} and {Re3Mo3S8} cores has been obtained and investigated. The clusters with different Re/Mo ratios were isolated as individual compounds, which made it possible to study their spectroscopic and electrochemical properties. The geometry of the new clusters was studied using a combination of X-ray diffraction analysis, XAS and quantum chemical DFT calculations. It was shown that the properties of the new clusters, such as the number and position of electrochemical transitions, electronic structure and change in geometry with a change in charge, are similar to the properties of clusters based on the {Re4Mo2Se8} and {Re3Mo3Se8} cores described earlier.


Introduction
Chemical and physical properties, as well as the application potential of octahedral cluster complexes based on {M 6 (µ 3 -Q) 8 } cores (M = Re, Q = S, Se, or Te; M = Mo or W, Q = Cl, Br, or I), attract great interest and have been intensively studied over the past few decades [1][2][3][4]. Within the {M 6 Q 8 } core, the octahedral M 6 metal cluster is surrounded by eight inner ligands Q, which are bound to the faces of the octahedron. Each metal atom is coordinated by one apical ligand L, forming a [{M 6 Q 8 }L 6 ] n cluster unit. The chemical composition of the {M 6 Q 8 } cluster core and, in particular, the nature of metal atoms largely determine the physical properties of the cluster-based compounds, such as their redox properties, phosphorescence, or solar light-harvesting performances [5][6][7][8][9]. The synthesis of heterometallic clusters based on M 6−x M x clusters with a controlled M/M ratio may lead to a smooth and predictable change in the properties of clusters. This is a promising approach for the design of cluster-based materials with predefined functionality. However, this approach may be quite difficult to realize since the octahedral cluster cores are usually formed during high-temperature synthesis and are chemically inert to the substitution of metal atoms at lower temperatures.
Solid-state Chevrel phases with the composition Mo 2 Re 4 Q 8 (Q = S, Se) were the first published examples of compounds based on heterometallic octahedral clusters. They were Molecules 2023, 28, 5875 2 of 18 obtained by the high-temperature reaction between stoichiometric quantities of simple substances [10]. Similar experimental protocols were used to prepare other {Mo 6−x M x Q 8 } heterometallic polymeric cluster-based phases (M = Re, Q = Te, x = 2; M = Ru, Q = Te, 1.5 ≤ x ≤ 2, Q = Se, x = 2; M = Rh, Q = Te, x = 0.5, 1.33) [11][12][13][14][15][16], as well as Cs 3 Re 5 OsS 11 [17]. Note that, for Mo-M -Q systems, well-defined compositions with integer x values (Mo 4  Soluble heterometallic octahedral clusters were obtained for the following metal combinations: rhenium/osmium [18][19][20], molybdenum/niobium [21] and rhenium/molybdenum [22,23]. All the compounds were obtained by high-temperature reactions between simple substances and binary precursors. Clusters with {Re 6−x Os x Se 8 } (x = 1-3) and {Mo 5 NbI 8 } cores can be isolated as individual species directly from the reaction mixtures. Meanwhile, clusters with {Re 6−x Mo x Q 8 } (Q = S, Se; x = 1-3) cores were formed as apparent singlephase solid solutions but contained simultaneously several cluster cores with the same charges and different values of x. The separation of the different cluster cores proved to be a non-trivial task since they have similar geometric parameters and charges and do not separate chromatographically. Very recently, the separation and detailed investigation of [Re 6−x Mo x Se 8 (CN) 6 ] n− cluster units (x = 1-3) were reported [24,25]. It was shown that the variation in the Re/Mo ratio led to drastic changes in the redox and spectroscopic properties of the clusters, which may allow for the obtaining of sensor and electrochromic materials based on these functionalized species. The aforementioned complicated nature of solid-state phases based on {Re 6−x Mo x Se 8 } units led us to re-investigate the "sulfide" solid-state phases formed as a result of the reactions between ReS 2 , MoS 2 and KCN and reported initially as constant composition phase K 6 [Re 3 Mo 3 S 8 (CN) 5 ] [23]. Recently, it was found that the synthesis of that chemical composition at different temperatures led to the formation of K 6 [Re 6−x Mo x S 8 (CN) 5 ] solid solutions with 2.75 < x < 3.63 [26]. Here we report on the separation procedures that allowed us to obtain salts of [Re 4 Mo 2 S 8 (CN) 6 ] n− and [Re 3 Mo 3 S 8 (CN) 6 ] n− clusters as individual species. Moreover, we report on the spectroscopic properties and crystal structure investigations and compare them with the properties of the corresponding {Re 4 Mo 2 Se 8 } and {Re 3 Mo 3 Se 8 } cluster cores.

Synthesis
The reaction of ReS 2 and MoS 2 with KCN at 700 • C for 7 days led to the formation of the polymeric compound K 6 [Re 3.16 Mo 2.84 S 8 (CN) 5 ] (1) [26]. The crystal structure of this compound was described earlier [15]. It is based on cluster units polymerized by means of bridging cyanide groups in trans-position to form chains. It should be noted that the ReS 2 and MoS 2 precursors used in the synthesis of 1 were obtained at 1000 • C. The preparation of metal sulfides at lower temperatures may lead to the formation of polymer K 6 [Re 6-x Mo x S 8 (CN) 5 ] with different Re/Mo ratio.
Compound 1 is insoluble in deaerated water. The treatment of the reaction products with boiling Ar-saturated H 2 O led to the dissolution of the KCN excess. The interaction of 1 with O 2 in an aerated KCN solution resulted in depolymerization accompanied by two-electron oxidation of the cluster core and the formation of the discrete cluster anion [Re 3.2 Mo 2.8 S 8 (CN) 6 ] 5− , which was isolated and crystallized as potassium salt 2. Note that the Re/Mo ratio in compound 2 was the same as in compound 1, confirming the cocrystallization of several clusters with different cores in the reaction mixture. To determine the composition of the mixture of clusters, compound 2 was converted to (Ph 4 P) + salt by mixing the aqueous solutions of 2 and Ph 4 PCl. The precipitate formed had the composition (Ph 4 P) 4 6 ] n− (Figure 1). To separate clusters with different Re/Mo ratios, an excess of Bu4NCl was added to the solution of compound 2 in H2O (Scheme 1). It was found that this reaction is pH-dependent. In the case of {Re4Mo2S8}, the addition of KOH can promote oxidation. Using a KOH solution, the pH value of the reaction mixture was adjusted to 10.5. The reaction mixture was left in a glass with air access for one week. As a result, a green crystalline precipitate of compound (Bu4N)4[Re4Mo2S8(CN)6] (4) was formed. To separate clusters with different Re/Mo ratios, an excess of Bu4NCl the solution of compound 2 in H2O (Scheme 1). It was found that th pH-dependent. In the case of {Re4Mo2S8}, the addition of KOH can promo Using a KOH solution, the pH value of the reaction mixture was adjusted reaction mixture was left in a glass with air access for one week. As a re crystalline precipitate of compound (Bu4N)4[Re4Mo2S8(CN)6] (4) was formed To separate clusters with different Re/Mo ratios, an excess of Bu 4 NCl was added to the solution of compound 2 in H 2 O (Scheme 1). It was found that this reaction is pHdependent. In the case of {Re 4 Mo 2 S 8 }, the addition of KOH can promote oxidation. Using a KOH solution, the pH value of the reaction mixture was adjusted to 10.5. The reaction mixture was left in a glass with air access for one week. As a result, a green crystalline precipitate of compound (Bu 4 N) 4  occurred, causing the precipitation of salt 4, which is insoluble in H 2 O. Further holding of the filtrate solution in air did not lead to the precipitation of other products; therefore, we can assume that quantitative separation of the cluster with {Re 4 Mo 2 S 8 } core took place. This was confirmed by the investigation of the solution of compound 4 by mass spectrometry. In the mass spectrum (Figure 2), all intense signals correspond to adducts of the [Re 4 Mo 2 S 8 (CN) 6 ] n− cluster. After the isolation of compound 4, the aqueous reaction mixture remained colored, indicating the presence of other cluster compounds. This colored species was successfully extracted with CH 2 Cl 2 , then the organic layer was separated, evaporated to dryness in air and dissolved in CH 3 CN. A solution of KSCN in CH 3 CN was added, causing the quick precipitation of brown powder of K 5 [Re 3 Mo 3 S 8 (CN) 6 ] (5) (Scheme 1). The aqueous solution of compound 5 was mixed with the aqueous solution of Ph 4 PCl, causing the immediate precipitation of compound 6. As in the case of compound 3, compound 6 is soluble in CH 3 CN, allowing it to be examined by ESI-MS. The main isotopic distribution peak sets found match well the {Re 3 Mo 3 S 8 }-based cluster adducts ( Figure 3). Therefore, the separation and selective isolation of clusters with the {Re 4 Mo 2 S 8 } and {Re 3 Mo 3 S 8 } cores were carried out. occurred, causing the precipitation of salt 4, which is insoluble in H2O. Furt the filtrate solution in air did not lead to the precipitation of other products; can assume that quantitative separation of the cluster with {Re4Mo2S8} co This was confirmed by the investigation of the solution of compoun spectrometry. In the mass spectrum ( Figure 2), all intense signals correspo of the [Re4Mo2S8(CN)6] n− cluster. After the isolation of compound 4, the aqu mixture remained colored, indicating the presence of other cluster com colored species was successfully extracted with CH2Cl2, then the organ separated, evaporated to dryness in air and dissolved in CH3CN. A solutio CH3CN was added, causing the quick precipitation of brown K5[Re3Mo3S8(CN)6] (5) (Scheme 1). The aqueous solution of compound 5 wa the aqueous solution of Ph4PCl, causing the immediate precipitation of com in the case of compound 3, compound 6 is soluble in CH3CN, allowing it to by ESI-MS. The main isotopic distribution peak sets found mat {Re3Mo3S8}-based cluster adducts ( Figure 3). Therefore, the separation isolation of clusters with the {Re4Mo2S8} and {Re3Mo3S8} cores were carried o   (4) Table 2). This correlation wa earlier for selenide heterometallic clusters [24]. M-Q and M-M are shorter for heterometallic clusters than for selenide ones, in accordance with the difference atomic radii of sulfur and selenium.   Table 2). This correlation was found earlier for selenide heterometallic clusters [24]. M-Q and M-M are shorter for sulfide heterometallic clusters than for selenide ones, in accordance with the difference in the atomic radii of sulfur and selenium.

Electrochemical Properties
The separation of the octahedral heterometallic clusters using the differences in their redox properties and solubility was first described for the clusters with {Re4Mo2Se8} and {Re3Mo3Se8} cores [24]. In that case, the process of precipitation of the  Table 1), respectively. In comparison with the [Re4Mo2S8(CN)6] n− clusters, the potentials of the corresponding transitions are shifted by 0.173 and 0.108 V to the anodic region (Table 3). This shift correlates well with the known electrochemical data for homometallic octahedral clusters of rhenium [Re6Q8(CN)6] 4−/3− , where a successive cathodic shift of the oxidation potentials occurs upon changing the nature of the inner ligands from S to Se and Te [27,28].

Electrochemical Properties
The separation of the octahedral heterometallic clusters using the differences in their redox properties and solubility was first described for the clusters with {Re 4 Mo 2 Se 8 } and {Re 3 Mo 3 Se 8 } cores [24]. In that case, the process of precipitation of the (Bu 4 N) 4 (Table 3). This shift correlates well with the known electrochemical data for homometallic octahedral clusters of rhenium [Re 6 Q 8 (CN) 6 ] 4−/3− , where a successive cathodic shift of the oxidation potentials occurs upon changing the nature of the inner ligands from S to Se and Te [27,28] (Table 3). One can notice that the nature of the inner chalcogenide ligand has much less impact on the electrochemical behavior of clusters with a higher content of molybdenum forming the cluster.     (Table 3). One can notice that the nature of the inner chalcogenide ligand has much less impact on the electrochemical behavior of clusters with a higher content of molybdenum forming the cluster.
One may see that embedding molybdenum atoms instead of rhenium ones within the {Re 6 } metal core increases the number of electrochemically accessible redox transitions of the resulting compounds and shifts them to the negative potential region (Table 3. The

Electronic Structure
In order to analyze the geometry and electronic structure of novel heterometallic cores {Re 4       The removal of electrons from HOMO causes a shift of HOMO-1 down in energy. The energy gap between SOMO and LUMO remains almost the same as the HOMO-LUMO gap for 24-electron anions. Further oxidation of the clusters causes a lowering of HOMO in energy. As a result, 22-electron clusters do not show a large gap between HOMO and the lower orbitals. On the contrary, the energy gap between HOMO and LUMO in 22-electron clusters (HOMO and HOMO-1 in 24-electron clusters) increases and reaches 0.5 eV. The gap between LUMO and LUMO+1 (HOMO-LUMO gap in 24-electron clusters) remains almost unchanged.

Geometry Optimization
According to the calculation data, the 24-electron metal core {Re 3 Mo 3 } is close to octahedron shape, especially for the mer-isomer and, to a lesser extent, for the fac-isomer, with little difference in M-M bond distances (Figure 8, 24 CSEs). The two-electron removal leads to a significant shortening of the Re-Re distances and an elongation of the Mo-Mo distances in the metal core in the case of both isomers (Figure 8, 22 CSEs). The distortion of the octahedron leads to C 2v and C s metal cores for merand fac-isomers, respectively. The difference between the longest and shortest metal-metal distances for the 22-electron mer-isomer is 0.133 Å (for the fac-isomer, 0.182 Å).

Geometry Optimization
According to the calculation data, the 24-electron metal core {Re3Mo3} is close to octahedron shape, especially for the mer-isomer and, to a lesser extent, for the fac-isomer, with little difference in M-M bond distances (Figure 8, 24 CSEs). The two-electron removal leads to a significant shortening of the Re-Re distances and an elongation of the Mo-Mo distances in the metal core in the case of both isomers (Figure 8, 22 CSEs). The distortion of the octahedron leads to C2v and Cs metal cores for mer-and fac-isomers, respectively. The difference between the longest and shortest metal-metal distances for the 22-electron mer-isomer is 0.133 Å (for the fac-isomer, 0.182 Å).  The resulting distortion cannot be determined from the structural data obtained by X-ray diffraction analysis since the symmetry of the cluster does not coincide with the symmetry of the structures, and the diffraction data contain the average positions of the metal atoms and, hence, the average interatomic distances. However, it was not possible to unambiguously determine by EXAFS the type of core isomerism from the obtained data due to the close values of the r-factor from fitting modeled data with different isomers (Tables S1-S4 in Supplementary Materials, r-factor values). A similar distortion character is observed for the {Re4Mo2} cluster, where two-electron removal also leads to the shortening of the Re-Re distances and the elongation of the Mo-Mo distances, giving C2v and D4h metal cores for cis-and trans-isomers, respectively (Figure 9).

Materials and Methods
All reagents and solvents were used as purchased. Elemental analysis was made on a EuroVector EA3000 analyzer (EuroVector, Pavia, Italy). Energy dispersive spectroscopy (EDS) was performed on a Hitachi TM3000 TableTop SEM (Hitachi, Ltd., Chiyoda City, Tokyo, Japan) with Bruker QUANTAX 70 EDS equipment (Bruker Corporation, Billerica, MA, USA). FT-IR spectra in KBr pellets were recorded on a Bruker Scimitar FTS 2000 spectrometer in the range 4000-375 cm −1 . UV-Vis absorption measurements were performed in the wavelength range of 400-800 nm on an Agilent Cary 60 spectrophotometer (Agilent Technologies, Inc., Santa Clara, CA, USA). Electrospray ionization mass spectrometry was performed on a Bruker maXis 4G high-resolution ESI-q-TOF spectrometer. The mass spectra were recorded under the following conditions: registration of negative ions from m/z = 600 Da to 4000 Da, voltage +2500 V, pressure on the nebulizer of 0.8 bar, drying gas flow of 4 L/min and drying gas temperature of 180 • C. Cyclic voltammetry was carried out on an Electrochemical Instruments Elins P-20 × 8 voltammetry analyzer (Electrochemical Instruments, Chernogolovka, Russia) using a three-electrode scheme with glassy carbon working, Pt auxiliary and Ag/AgCl/3.5M KCl reference electrodes. Investigations were carried out for 2.5·10 −3 M solution of cluster salts 3 and 6 in a 0.1 M solution of Bu 4 NClO 4 in CH 3 CN or (CH 3 ) 2 O, respectively, under an Ar atmosphere. The registered value of E 1/2 for the Fc 0/+ couple was 0.440 V in the same conditions. Powder X-ray diffraction (PXRD) data were collected using a Philips PW 1820/1710 diffractometer (Cu, Kα radiation, graphite monochromator, Si as external reference).

Synthesis of Compounds
ReS 2 and MoS 2 were synthesized by the reaction between stoichiometric amounts of corresponding metal and sulfur in evacuated silica ampoules at 1000 • C for 48 h. Phase purity of binary precursors was confirmed by PXRD.
Preparation of K 6 [Re 3.2 Mo 2.8 S 8 (CN) 5 ] (1). Compound 1 was prepared using a technique derived from that previously described [15]. ReS 2 (1.02 g, 4 mmol), MoS 2 (0.65 g, 4 mmol) and KCN (1.68 g, 25 mmol) were mixed in a silica ampoule. The ampoule was evacuated and sealed. The reaction was carried out at 700 • C for 8 days, and then the ampoule was cooled down to room temperature in 12 h. The reaction mixture was stirred in Ar-saturated H 2 O (250 mL) under Ar gas flow at room temperature for 1 h to remove unreacted KCN and was filtered off. The resulting solid residue was washed on a glass filter with an EtOH/H 2 O mixture (7/1 vol., 2 portions of 20 mL) and EtOH (30 mL). The mixture containing octahedral black crystals of 1 with an admixture of unreacted metal sulfides was separated by sonication in EtOH with subsequent decantation, and then product 1 was dried in air. Yield: 1.30 g (66% based on total amount of metals). EDS: K:Re:Mo:S = 5.7:3.2:2.8:8.3. FT-IR (KBr, cm −1 ): 2088 (C≡N). Phase purity of the product was confirmed by PXRD. PXRD also revealed that compound 1 is isostructural to the K 6 [Re 3 Mo 3 S 8 (CN) 5 ] phase reported previously [15].

Single-Crystal Diffraction Studies
Single-crystal X-ray diffraction data were collected at room temperature on an APEX II Bruker AXS diffractometer using a Mo-Kα X-ray wavelength (λ = 0.71073 Å) and processed with the APEX 2 program suite [29]. Frame integration and data reduction were carried out with the program SAINT [30]. The program SADABS [31] was employed for multi-scan absorption corrections. The structures of compounds 4 and 5 were solved by direct methods using the SHELXT program [32] and refined with full-matrix least-squares methods based on F 2 (SHELXL) [33] with the aid of the WinGX platform [34]. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. The statistical distribution of rhenium and molybdenum atoms on their respective crystallographic sites was considered by constraining equivalent atomic coordinates and anisotropic atomic displacement parameters. From preliminary refinements, a Re/Mo ratio close to 4/2 was refined for 4 and close to 3/3 for 5. Considering that these ratios are in agreement with elemental compositions determined from experimental chemical analyses, final refinements were conducted by using the SUMP command, restraining a Re/Mo ratio of 4/2 for 4 and of 3/3 for 5. This restriction has no influence on the reliability factors or the largest difference in peak and hole values. Hydrogen atoms were included in the structural model considering their calculated positions, and their equivalent isotropic displacement parameters were constrained to be equal to 1.5 times that of the linked atom for methyl hydrogens and 1.2 times for others. Hydrogen atoms in the water molecules were not localized. Publishing data were computed using CRYSCALC program [35]. Selected crystal, collection and refinement data for 4 and 5 are gathered in Table 6. Selected bond distances are listed in Table 2 6 ] n− cluster anions (n = 4-6) and granand mer-[Re 3 Mo 3 Se 8 (CN) 6 ] n− cluster anions (n = 5-7) were carried out in the ADF2021 program package [36,37]. Geometric parameters were optimized with the PW92+revPBE density functional [38,39] and all-electron TZ2P basis set [40]. The zero-order regular approximation (ZORA) [41] for the scalar relativistic effects and the Conductor-like Screening Model (COSMO) [42] for water environment were used in all calculations. Spin-unrestricted calculations were used for cisand trans-[Re 4 Mo 2 Se 8 (CN) 6 ] 5− and granand mer-[Re 3 Mo 3 Se 8 (CN) 6 ] 6− cluster anions containing 23 CSEs (one unpaired electron). Calculations were performed using C 1 symmetry. In order to facilitate comparisons with similar works in the literature, MO units are a.u. instead of e 1/2 ·Å −3/2 , which is common practice in coordination chemistry [43,44].

EXAFS
X-ray absorption spectroscopy (XAS) measurements were carried out at room temperature in transmission mode at the K edge of Mo and L 3 -absorption edge of Re at the beamline SAMBA [45] in the Soleil Synchrotron, France (proposal 20210623). Radiation coming from a bending magnet source was monochromatized by a Si(220) fixed-exit sagittally focusing double-crystal monochromator. Harmonic rejection was performed using a couple of mirrors that also focused the monochromatic beam vertically (spot area around 300 × 300 µm 2 ). The samples were examined as self-supporting pellets (matrix cellulose), for which the amount of sample was optimized in order to have a proper XAS signal. The extended X-ray absorption fine structure (EXAFS) signal treatment was performed according to standard procedures: subtraction of the pre-edge and post-edge backgrounds, edge normalization and extraction of EXAFS signal χ(k) and its Fourier transformation, which provides a map in the real space of the distribution of the distances R around the absorber atom. The Demeter software package was used to perform data treatment and fitting [46] using the phase and amplitude calculated using the FEFF-10lite code [47]. Structure models for the {Re 3 Mo 3 S 8 } and {Re 4 Mo 2 S 8 } cores were obtained from crystallographic data. The occupancies of the metal positions were changed so that the rhenium and molybdenum atoms were in different positions, corresponding to the isomerism of the core ( Figures S1 and S4 in Supplementary Materials). EXAFS measurements and Fourier transform magnitudes for samples 4 and 5 are given in Figures S2, S3, S5 and S6 in the Supplementary Materials, respectively. The final refinement parameters are given in Tables S1-S4 in the Supplementary Materials.

Conclusions
We have demonstrated the possibility of separating anionic complexes based on {Re 4  } clusters shows that the charge states and spectroscopic properties of heterometallic clusters are determined by the ratio of rhenium and molybdenum atoms in the core to a much greater extent than by the type of internal chalcogenide ligands. Such anionic complexes could be further used as building blocks for the design of photoelectrodes for solar cells. Homometallic clusters have already been shown to be promising solar cell components. For example, FTO film coated by the Cs 2 [{Mo 6 I 8 }I 6 ] complex displayed extremely rare ambipolar semiconducting properties, i.e., the ability to generate both holes and electrons as main charge carriers [8]. According to the published data, molybdenum cluster complexes have an energy gap of 1.9 eV, which is close to the values for heterometallic clusters-2.2 eV for {Re 3 Mo 3 S 8 } and 2.0 eV for {Re 4 Mo 2 S 8 }. Materials based on (Bu 4 N) 3 [{Re 6 Q 8 }Cl 6 ] (Q = S, Se) cluster complexes also exhibit ambipolar properties [9]. The energy gaps for these clusters are 2.1 and 1.9 eV for Q = S and Se, respectively. The deposition of the clusters on the FTO surface has little effect on the energy gaps. According to the published data, we assume that heterometallic cluster complexes have similar semiconductor properties but may have different optical properties in comparison with homometallic clusters.